fix(backend): migrate PostgreSQL traffic columns from int4 to bigint (#290)

## Summary

- Widens legacy PostgreSQL traffic/quota columns from `integer` to
`bigint` to prevent int4 overflow
- Fixes federation share creation failure when traffic limits exceed 2GB
(e.g., `536870912000` bytes = 500GB)
- Bumps schema version from 4 to 5 with auto-migration on backend
startup

## Affected Tables

- `user`: `flow`, `in_flow`, `out_flow`
- `forward`: `in_flow`, `out_flow`
- `statistics_flow`: `flow`, `total_flow`
- `tunnel`: `flow`
- `user_tunnel`: `flow`, `in_flow`, `out_flow`
- `peer_share`: `max_bandwidth`, `current_flow`

## Test Plan

- ✅ Unit tests added for migration execution and error handling
- ✅ Contract tests passed
- ✅ Repository tests passed

Commands:
```bash
cd go-backend && go test ./internal/store/repo/...
cd go-backend && go test ./tests/contract/...
```

go-backend/internal/store/repo/repository.go

@@ -2612,11 +2612,12 @@ func (r *Repository) GetUserTunnelByID(id int64) (*model.UserTunnel, error) {
 
 // ─── Migration ───────────────────────────────────────────────────────
 
-const currentSchemaVersion = 4
+const currentSchemaVersion = 5
 
 var ensurePostgresIDDefaultsFn = ensurePostgresIDDefaults
 var migrateViteConfigValueColumnTypeFn = migrateViteConfigValueColumnType
 var migrateSpeedLimitTunnelBindingFn = migrateSpeedLimitTunnelBinding
+var migratePostgresTrafficInt64ColumnsFn = migratePostgresTrafficInt64Columns
 
 func getSchemaVersion(db *gorm.DB) int {
 	var v model.SchemaVersion
@@ -2680,6 +2681,12 @@ func migrateSchema(db *gorm.DB) error {
 		}
 	}
 
+	if ver < 5 {
+		if err := migratePostgresTrafficInt64ColumnsFn(db); err != nil {
+			return err
+		}
+	}
+
 	setSchemaVersion(db, currentSchemaVersion)
 	return nil
 }
@@ -2744,6 +2751,87 @@ func migrateSpeedLimitTunnelBinding(db *gorm.DB) error {
 	return nil
 }
 
+func migratePostgresTrafficInt64Columns(db *gorm.DB) error {
+	if db == nil {
+		return errors.New("nil db")
+	}
+
+	if db.Dialector.Name() != "postgres" {
+		return nil
+	}
+
+	type trafficColumn struct {
+		TableName  string
+		ColumnName string
+	}
+
+	columns := []trafficColumn{
+		{TableName: "user", ColumnName: "flow"},
+		{TableName: "user", ColumnName: "in_flow"},
+		{TableName: "user", ColumnName: "out_flow"},
+		{TableName: "forward", ColumnName: "in_flow"},
+		{TableName: "forward", ColumnName: "out_flow"},
+		{TableName: "statistics_flow", ColumnName: "flow"},
+		{TableName: "statistics_flow", ColumnName: "total_flow"},
+		{TableName: "tunnel", ColumnName: "flow"},
+		{TableName: "user_tunnel", ColumnName: "flow"},
+		{TableName: "user_tunnel", ColumnName: "in_flow"},
+		{TableName: "user_tunnel", ColumnName: "out_flow"},
+		{TableName: "peer_share", ColumnName: "max_bandwidth"},
+		{TableName: "peer_share", ColumnName: "current_flow"},
+	}
+
+	for _, column := range columns {
+		if err := alterPostgresColumnToBigIntIfNeeded(db, column.TableName, column.ColumnName); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func alterPostgresColumnToBigIntIfNeeded(db *gorm.DB, tableName, columnName string) error {
+	if db == nil {
+		return errors.New("nil db")
+	}
+
+	if tableName == "" || columnName == "" {
+		return errors.New("empty table or column name")
+	}
+
+	type columnRow struct {
+		DataType string `gorm:"column:data_type"`
+	}
+
+	var row columnRow
+	if err := db.Raw(
+		`SELECT data_type FROM information_schema.columns
+		 WHERE table_schema = current_schema()
+		   AND table_name = ?
+		   AND column_name = ?`,
+		tableName, columnName,
+	).Scan(&row).Error; err != nil {
+		return fmt.Errorf("inspect %s.%s type: %w", tableName, columnName, err)
+	}
+
+	if row.DataType == "" || strings.EqualFold(row.DataType, "bigint") {
+		return nil
+	}
+	if !strings.EqualFold(row.DataType, "integer") {
+		return nil
+	}
+
+	if err := db.Exec(fmt.Sprintf(
+		"ALTER TABLE %s ALTER COLUMN %s TYPE BIGINT",
+		quoteSQLIdentifier(tableName),
+		quoteSQLIdentifier(columnName),
+	)).Error; err != nil {
+		return fmt.Errorf("alter %s.%s to bigint: %w", tableName, columnName, err)
+	}
+
+	return nil
+}
+
 func ensurePostgresIDDefaults(db *gorm.DB) error {
 	if db.Dialector.Name() != "postgres" {
 		return nil

go-backend/internal/store/repo/repository_migrate_test.go

@@ -3,6 +3,7 @@ package repo
 import (
 	"database/sql"
 	"errors"
+	"strings"
 	"testing"
 
 	gsqlite "github.com/glebarez/sqlite"
@@ -250,3 +251,115 @@ func TestMigrateSchemaClearsSpeedLimitTunnelBinding(t *testing.T) {
 		t.Fatalf("expected schema version %d, got %d", currentSchemaVersion, schemaVersion)
 	}
 }
+
+func TestMigrateSchemaRunsTrafficInt64MigrationForLegacySchema(t *testing.T) {
+	db, err := gorm.Open(gsqlite.Open(":memory:"), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Silent),
+	})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	t.Cleanup(func() {
+		sqlDB, _ := db.DB()
+		if sqlDB != nil {
+			_ = sqlDB.Close()
+		}
+	})
+
+	if err := db.Exec(`CREATE TABLE schema_version (version INTEGER NOT NULL DEFAULT 0)`).Error; err != nil {
+		t.Fatalf("create schema_version: %v", err)
+	}
+	if err := db.Exec(`INSERT INTO schema_version(version) VALUES(?)`, 4).Error; err != nil {
+		t.Fatalf("seed schema_version: %v", err)
+	}
+
+	originalIDRepair := ensurePostgresIDDefaultsFn
+	ensurePostgresIDDefaultsFn = func(db *gorm.DB) error {
+		return nil
+	}
+	t.Cleanup(func() {
+		ensurePostgresIDDefaultsFn = originalIDRepair
+	})
+
+	called := 0
+	originalMigrate := migratePostgresTrafficInt64ColumnsFn
+	migratePostgresTrafficInt64ColumnsFn = func(db *gorm.DB) error {
+		called++
+		return nil
+	}
+	t.Cleanup(func() {
+		migratePostgresTrafficInt64ColumnsFn = originalMigrate
+	})
+
+	if err := migrateSchema(db); err != nil {
+		t.Fatalf("migrateSchema: %v", err)
+	}
+
+	if called != 1 {
+		t.Fatalf("expected traffic bigint migration to run once, got %d", called)
+	}
+
+	var schemaVersion int
+	if err := db.Raw(`SELECT version FROM schema_version LIMIT 1`).Row().Scan(&schemaVersion); err != nil {
+		t.Fatalf("query schema_version: %v", err)
+	}
+	if schemaVersion != currentSchemaVersion {
+		t.Fatalf("expected schema version %d, got %d", currentSchemaVersion, schemaVersion)
+	}
+}
+
+func TestMigrateSchemaReturnsTrafficInt64MigrationError(t *testing.T) {
+	db, err := gorm.Open(gsqlite.Open(":memory:"), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Silent),
+	})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	t.Cleanup(func() {
+		sqlDB, _ := db.DB()
+		if sqlDB != nil {
+			_ = sqlDB.Close()
+		}
+	})
+
+	if err := db.Exec(`CREATE TABLE schema_version (version INTEGER NOT NULL DEFAULT 0)`).Error; err != nil {
+		t.Fatalf("create schema_version: %v", err)
+	}
+	if err := db.Exec(`INSERT INTO schema_version(version) VALUES(?)`, 4).Error; err != nil {
+		t.Fatalf("seed schema_version: %v", err)
+	}
+
+	originalIDRepair := ensurePostgresIDDefaultsFn
+	ensurePostgresIDDefaultsFn = func(db *gorm.DB) error {
+		return nil
+	}
+	t.Cleanup(func() {
+		ensurePostgresIDDefaultsFn = originalIDRepair
+	})
+
+	wantErr := errors.New("traffic bigint migration failed")
+	originalMigrate := migratePostgresTrafficInt64ColumnsFn
+	migratePostgresTrafficInt64ColumnsFn = func(db *gorm.DB) error {
+		return wantErr
+	}
+	t.Cleanup(func() {
+		migratePostgresTrafficInt64ColumnsFn = originalMigrate
+	})
+
+	err = migrateSchema(db)
+	if !errors.Is(err, wantErr) {
+		t.Fatalf("expected error %v, got %v", wantErr, err)
+	}
+}
+
+func TestAlterPostgresColumnToBigIntIfNeededValidatesNames(t *testing.T) {
+	if err := alterPostgresColumnToBigIntIfNeeded(nil, "peer_share", "max_bandwidth"); err == nil || !strings.Contains(err.Error(), "nil db") {
+		t.Fatalf("expected nil db error, got %v", err)
+	}
+	if err := alterPostgresColumnToBigIntIfNeeded(&gorm.DB{}, "", "max_bandwidth"); err == nil || !strings.Contains(err.Error(), "empty table or column name") {
+		t.Fatalf("expected empty name error, got %v", err)
+	}
+	if err := alterPostgresColumnToBigIntIfNeeded(&gorm.DB{}, "peer_share", ""); err == nil || !strings.Contains(err.Error(), "empty table or column name") {
+		t.Fatalf("expected empty name error, got %v", err)
+	}
+}

plans/019-federation-share-traffic-bigint-migration.md

@@ -0,0 +1,21 @@
+# 019 Federation Share Traffic Bigint Migration
+
+## Checklist
+
+- [x] Inspect federation share creation failure and identify the PostgreSQL `int4` overflow source.
+- [x] Audit other traffic-related legacy PostgreSQL columns that may still be `integer` despite Go models using `int64`.
+- [x] Add a schema migration that widens legacy traffic/quota columns from `integer` to `bigint`.
+- [x] Add migration tests covering the new schema version branch and error propagation.
+- [x] Run focused backend verification for the migration changes.
+
+## Notes
+
+- The reported failing value `536870912000` is 500 GiB in bytes and overflows PostgreSQL `int4`.
+- The fix widens historical PostgreSQL traffic columns in `user`, `forward`, `statistics_flow`, `tunnel`, `user_tunnel`, and `peer_share` to `BIGINT` when needed.
+
+## Test Record
+
+- Command: `cd go-backend && go test ./internal/store/repo/...`
+- Result: passed.
+- Command: `cd go-backend && go test ./tests/contract/...`
+- Result: passed.